This invention relates generally to the field of subsurface environmental remediation of underground sites contaminated with volatile compounds, and more specifically to equipment for measuring the effective diffusion coefficient and gas-phase permeability of a vapor moving through a porous media.
Volatile organic compounds (VOCs), also known as Non-Aqueous Phase Liquids (NAPLs), are the principal contaminants at many commercial and DOE sites. Some examples of VOCs include compounds such as aromatic hydrocarbons (e.g., benzene, toluene, xylenes); halogenated/chlorinated hydrocarbons (e.g., trichloroethylene (TCE), carbon tetrachloride); alcohols, ketones (e.g., acetone) and aliphatic hydrocarbons (e.g., hexane, octane). Other VCs of interest to groundwater protection include methyl tert-butyl ether (MTBE), other gasoline additives, toluene, and xylene (See 40 CFR 141.32 Primary Drinking Water Standards). Volatile contaminants can also include toxic chemicals, volatile pesticides, volatile fertilizers, buried volatile explosives, and organic compounds with low volatility. VC's can include gases or vapors other than volatile organic compounds, such as nitrogen oxide, nitrous oxide, carbon monoxide, carbon dioxide, hydrogen gas, and toxic gases, such as ammonia, chlorine, phosphonates, nerve gas (mustard, sarin, VX).
Tens of thousands of sites containing toxic chemical spills, leaking underground storage tanks, and chemical waste dumps require characterization and long-term monitoring (stewardship) to protect environmental resources (e.g., groundwater) and to determine when remedial measures are needed. Current methods are costly and time-intensive, and limitations in sampling and analytical techniques exist. For example, the Department of Energy (DOE) Savannah River Site requires manual collection of nearly 40,000 groundwater samples per year, which can cost between $100 to $1,000 per sample for off-site analysis (not including the cost of collecting the samples). Numerous commercial sites and applications, which include over two million underground storage tanks (e.g., at gas stations), also require monitoring to satisfy EPA requirements; as well as thousands of commercially contaminated sites that require characterization, monitoring, and/or remediation. Also, oil and natural gas fields currently take individual fluid samples manually from wells at a cost of nearly $250,000 per sample.
An important aspect of site characterization is understanding how vapors from volatile compounds move through porous soil, sand, cracked rock, etc, at various temperatures and moisture levels, and with or without a forced flow of a purge gas. A variety of computer codes exist that can model the flow of gases through a porous medium. However, the physical properties that are used in these codes, such as vapor diffusion coefficients and gas-phase permeabilities, are often unknown due to a lack of relevant experimental data.
Currently, there is no known portable device that can rapidly and accurately measure the effective vapor diffusion coefficient and gas-phase permeability for a vapor moving through a porous media, including advection by flow of a purge gas. A need exists, therefore, for an easily portable device that may be carried and used in the field, e.g., at an environmental restoration site. These sites commonly have core samples taken from both the contaminated and uncontaminated subsurface regions of the site. However, rather than mailing a large number of samples to an off-site laboratory, it would be faster and cheaper to analyze the samples on site using such a portable diffusion coefficient meter. Additionally, the properties of the subsurface media may change during the course of environmental remediation, that could require ongoing measurements of these properties.
A desirable aspect of in-situ monitoring is having the capability to not only detect the presence of contaminants, but to also characterize the contaminant in terms of its composition and location of its source. Traditional monitoring techniques require that the monitoring device be in the immediate vicinity of the contaminant to accurately detect and identify the contaminant location. Hence, a need exists for a characterization method that can identify the source's location using one or more in-situ sensors that are located relatively far away from the source.
If the effective vapor diffusion coefficient, D, is known, then analytical solutions may be used to estimate the distance from the in-situ sensor to the source. A portable diffusion coefficient meter could be used to measure the effective vapor diffusion coefficient, D, of actual core samples in the field, thereby increasing the accuracy and speed for which such a calculation (of the source distance) may be made. A triangulation method may be used with multiple in-situ sensors to determine the source location in two- and three-dimensions.
Current subsurface remediation methods use soil vapor extraction (SVE) techniques in the vadose zone (e.g., using a vacuum pump to pull vapors out of an extraction/exhaust well and/or air sparging techniques in the saturated zone e.g., pumping air down into a well to below the liquid level to force (i.e., advect) the flow of contaminant vapors through porous soil in the vadose zone towards an exhaust (i.e., exit) well or other opening).
The effectiveness and efficiency of these remediation techniques rely strongly on how quickly vapors from the volatile contaminant can diffuse or be advected through the various porous subsurface zones. The feasibility and economics of undertaking such a remediation effort depends strongly on the effective diffusion coefficients and gas-phase permeabilities of the porous media (which may be heterogenous). Knowledge of these properties during the project can help understand any unexpected changes in the vapor extraction rates, etc. Finally, the ability to efficiently conduct remedial measures in real-time (as needed) can improve public confidence in the ability of federal, state, and local governmental agencies to protect the environment and prevent contaminant migration away from these contaminated sites.
Against this background, the present invention was developed.